Apparatus for stirring liquid materials, including culture mediums, have long been known. For example, Scharf et al. U.S. Pat. No. 3,649,465 provides a flask or vessel having an opening at its upper portion, with a closure for the opening, having a spindle extending therethrough, the spindle at its lower end having a magnetic stirrer, with a shroud extending in surrounding relationship to the spindle. The stirrer is driven magnetically by a driving magnet, and the magnetic stirrer, located at the bottom of the flask, may be adjusted through a limited vertical range by vertically adjusting the spindle within the limits permitted by the shroud.
Harker et al. U.S. Pat. No. 2,958,517 provides a flask having a rod guided in a bearing in a closure for the flask, the rod having at its lower end a magnetic impeller, which engages the bottom of the flask, the magnetic impeller being driven by a magnetic stirring apparatus on which the flask is held, the apparatus including an electric motor having a shaft driving a magnet which is magnetically coupled to the magnet within the flask.
Harker U.S. Pat. No. 3,572,651 provides a flask having a closure provided with a bearing on its underside, the bearing supporting a spindle having at its lower end, near the bottom of the flask, a magnetic stirrer, the magnetic stirrer being driven by a conventional magnetic driving apparatus.
Mazowski U.S. Pat. No. 3,622,129 also discloses a magnetic stirrer apparatus, in which a flask has an opening, a closure for the opening with a rod extending through the opening, the rod supporting at its lower end a magnetic stirrer, and the rod being adjustable, vertically, in the closure, so as to position the stirrer at different depths in the liquid in the flask.
Sada et al. U.S. Pat. No. 4,310,253 discloses an apparatus in which a vessel containing a body of liquid has floating, magnetic particles which are caused to rotate by a rotating magnetic field, to rotate the interface between, for example, a liquid and a gaseous body in the vessel.
There have been provided stirrer apparatus, particularly for cell culture stirring, in which a linearly extending buoyant stirrer was provided, having magnetic means associated with it, so that the stirrer could be rotated by a rotating magnetic field. Such apparatus, while having many advantages, presented a problem of vertical instability when the liquid level was low. Additionally, the stirrer could wander in the flask or vessel when stirring is interrupted for sampling, or by loss of synchronization with the driving magnetic field.
There are also known in the prior art various magnetic stirrer apparatus constructions in which magnetic field generating elements, such as coils, were positioned laterally of a vessel, to drive a rotor located within, specifically at the bottom, of the vessel: in Stringham U.S. Pat. No. 1,242,493, the rotor was, itself, the stirrer element, while in Stainbrook U.S. Pat. No. 1,420,773, the rotor was connected to a shaft which caused rotation of a stirrer element, generally in the form of a propeller.
The prior art in which the stirrer is submerged in the liquid was subject to various defects and deficiencies, including constructions which were difficult to clean and which did not have sufficient cell proliferating action. In some cases, obstructions were provided to the liquid motion by the stirrers, or stirring action unsuitably vigorous for cell culture was required to insure complete stirring action. Those constructions in which a stirrer is submerged, also provide difficulty in that the stirring action is non-uniform with various amounts of liquid in the vessel, and are therefore unsuitable where a culture medium stirring or agitating is to be provided.
Culture medium provides for the growth of cells, from nutrients contained in the medium. The stirring action required is not violent, but is gentle, and care must be taken to avoid damage to cells, such as by violent agitation, and by crushing.